Indicating instrument



' Feb. 22, 1927.

' L. F. BLUME' 'INDICATING INSTRUMENT Filed Nov. 12,- 1924 CENTIGRADE Inventor: Louis ITBlume,

His Attorney.

Patented Feb. 22, 1927.

innate tries.

LOUIS IE. BLUE/2E, OE PITTSFIELD,

MASSAGHUSETTSg ASSIGNQB T GEI VEBAL ELEC- TRIC COMIANY, A CQRPORATION OF NEW YORK. I

' INDIGATING INSTRUMENT.

Application filed-November 12, 1924. Serial No. 749,548.

My invention relates to indicating instruments for use in connection with oil-imless wide ranges, however, with the result that the maximum load which the transformer can safely carry will also vary so as to be sometimes more and sometimes less thanthat indicated by its specified capacity rating. It is often important or desirable to know just what limit of load may be considered safe for a transformer to carry under conditions existing at the time so that it may be used to the greatest advantage. The general object of the invention is .to

provide an instrument for indicating in some suitable and convenient units the limit of load which a transformer may carry under conditions existing at the time.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Fig.1 shows an oil-immersed transformer equipped with an indicating instrument in accordance with the invention, part of the transformer casing being broken away to reveal the arrangement of parts inside; Fig. 2 instrument with its pointer and scales.

In the particular embodiment of the invention shown in the drawing, an indicating instrument 1 is conveniently mounted fon the casing 2 ofa transformer 3 immersed in a body of cooling and insulating oil 4. The indicating instrument 1 is of the usual thermometer type commonly employed for indicating the temperature of transformer oil and is connected by a tube 5 with a bulb 6 immersed in the oil near the top of the casing 2 wherethe temperature of the oil is highest during operation of the trans former. The face of the instrument, as

shown in Fig. 2," is provided with a pointer" 7 arranged to travel over a temperature scale 8 which is graduated in centigrade deshows'the'face of the indicating pointer 7 travels asit determined, as has been seen,

grees. Under the influence of temperature changes of the oil, the expansion and contraction of a suitable fluid int-he bulb 6 actuates the pointer 7 to indicate at any time on the scale 8 the temperature of the oil surrounding the bulb.

Now thetemperature of the oil alone is not a suificient safeguard against operating the transformer with excessive internal temperatures because the temperature of the hottest part of the transformer is equal to the temperature of the hottest oil only after the transformer has been unloaded so long that the two temperatures have had timeto become'equalized. As soon as any load is taken from the transformer, the windings are quickly heated to temperatures above that of the hottest oil. Even with 'an ex cessive load which may beheating parts'of the windings to dangerously high temperatures; it may bepossible for the temperature of the hot-testoil to be moderately 'lowbecause of low temperature of the cooling medium employed. This cooling medium may be the surrounding air as in the case of self-cooled transformers orit may be wa-' ter or both Water and air as in the case of water-cooled transformers",

For any given hottest oil temperature, I have found that the permissible orrmaximum safe load which the transformer can carry without exceeding a specified safe maximum temperature is definite and fixed. So, for every temperature which the hottest oil may assume, it is possible to determine the corresponding permissible or maximum safe load, either by theoretical calculation or bysimple test with suitable instruments. In accordance with. the present invention, therefore, a scale 9, which may be called a permissible load scale; is provided on the face of the instrument 1 .and over which the moves in response to changes in the temperature of the hottest oil surrounding the bulb 6. The permissibleload scale 9 1s graduatedin any suitable load units to indicate the respective permissible or maximum safe loads for the corresponding positions of the pointer'7, the position of the point-er at any time being p by the temperature of the hottest oil. Each temperasafe load indication on the scale 9. If de-f 'ture indication on the scale 8 is thus directly opposite the corresponding maximum 7 sired, the temperature scale 8 may be omitted and the pointer 7 will indicate at all times on the scale 5) the limit of safe load which thet-ransformer can carry.

In Fig. 2 ofthe drawing, the scale 9 is graduated to indicate the limit of safe load in percentage of the normal full load for which the transformer is designed under normal average conditions. The scale 9 may he graduated, however, in any other suitable load units suchv as in amperes or kilovolt-amperes.

The particular relationship of the values on the scales 8 and 9 as shown in Fig. 2 is of course not definitely fixed for all cases but will vary for different transformers and this relationship will also vary with the maximum temperature which may be con.- sidered safe for the hottest part of the windings of any given transformer. It is commonly considered that ninety-live degrees Centigrade is the maximum sai'c ten'ipcra ture for the hottest part of the windings of a transformer. Assuming: that this is true with a particular transformer for which the instrument indicated in the drawing designed and calculated, then it will he seen that when the oil ten'i iierature is eighty dcg'rees centigrade, the transforu'ier can carry a maximum load equal to its full load rating: (or 100% of its rated full load) without heating the windings above ninety-live degrees centigrade. If the temperature of the oil is greater than eighty degrees centigrade,

then the load must be decreased in accord ance with the position of the pointer 7 on the load scale 9 to prevent Wei-heating of the windings of the transformer. If the teiu 'icrature of the oil is less than eighty degrees centigrade. then the load may be increased, it desired to any value up to that indicated by the pointer 7 on the load scale 1 without danger of overheating the windings.

The invention provides a very simple and approximatclv accurate means for indicatingat a glance the limit of load which a transformer may safely carr} and it enables an operator to obtain from his transiimirmers their grcati'ist possible output without dangel of overloading them.

What I. claim asneiv and desire to secure by Letters Patent of the United. States, is,- 1. An indicating instrument for an oilimmersed transformer. said instrument com: prising a pointer operable in response to temperature changes of the oil, and a scale over which said pointer travels, said scale being); graduated in load units to indicate For each position of the pointer the limitot safe load which the transforn'ier can carry.

2. Au indicating instrument for an oilimmersed truns'lornier. said instrument comprising a scale graduated in temperature units. a pointer operable in response to tour perature changes oi the oil and arranged to travel over said scale to indicate the te1njlt'il'iltlll'c oi' the oil, and asecond scale over which said pointer is also arranged to travel. said second scale being graduated in load unitsv to indicate for each position oi. the pointer the limit of safe load which the transformer can carry,

In witness whereoifill have hereunto set my hand this tenth day of November. 1924.

LOUIS F. BLUME. 

